Photos are uploaded to iCloud, and downloaded to the Photos app on the Mac.

Plug in iPhone - Photos lets you import photos that haven't already been imported, or import photos that have already been imported again. And there is an option to delete items after import.

I don't want to import items that have already been imported. But I do want to delete photos from my iPhone (16 gigabytes simply isn't enough space any more – anyone considering buying a new phone, take note!)

Turns out, you can delete photos from your iPhone pretty easily using one of OSX's built-in apps called Image Capture.